1. The Field of the Invention
The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention is directed to novel bonding pads for integrated circuit devices for improved reliability, and methods for forming the same.
2. The Relevant Technology
In conventional semiconductor technology, bonding pads are formed in an integrated circuit as part of a top metal layer. The top metal layer is deposited and then patterned to form runners and bonding pads. Passivation layers are then deposited over the top metal layer and patterned, leaving openings in the passivation layers over the bonding pads. Later, during packaging of individual die, each bonding pad has an end of a wire bonded thereto through application of heat, pressure, sonic energy, other forms of energy, or a combination thereof An opposite end of each wire is bonded to an inner portion of a package lead.
The reliability of the bonding process is particularly critical since the bonding process occurs so late in the production cycle. Die being packaged have typically already been tested and sorted. Any problems in the wire bonding process thus impact only good die, and do so only after substantial investment in the production thereof.
Secure, reliable bonding of the wire to the bonding pad requires that the bonding pad be formed of metals compatible with the bonding process. Aluminum and aluminum alloys are typically employed to achieve the most reliable bonds. Reliable bonding further requires a certain minimum bonding pad thickness, so that sufficient material is present to form a secure bond.
An undesirable effect of the current technology is that the above requirements for reliable wire bonding effectively limit the minimum feature size of the top metal layer. Metal layers having sufficient thickness for reliable bonds are typically too thick to be easily patterned at higher resolutions. A relatively thick metal layer requires a relatively thick layer of photoresist to withstand the longer etch required to remove the thick metal layer. But a very high resolution exposure is generally obtained only in conjunction with a shallow depth of focus which is insufficient to provide high resolution exposure throughout a thick photoresist layer. Also, as the aspect ratio of the metal lines increases, the difficulty of adequately cleaning and filling between the lines, resulting in decreasing reliability with increasing thickness. Further, aluminum and aluminum alloys are more susceptible to electromigration than some other metals, which prevents the use of very thin metal lines.